1. Field of the Invention
The present invention relates to an acousto-optic tunable filter in which a source light beam is incident on the surface of a crystal body at an oblique angle.
2. Description of the Prior Art
In general, spectroscopes, spectrophotometers and the like are used in spectroscopy to obtain a spectrum by measuring the intensity of a source light beam in different wavelength regions. Prism spectroscopes and diffraction grating spectroscopes have been widely used for such a purpose. However, the acousto-optic tunable filter, abbreviated as AOTF, has spread recently, because of its high-speed and a seismic processing. In the AOTF, an acoustic wave is applied to a crystal body consisting in a uniaxial crystal material such as a tellurium dioxide (TeO.sub.2) crystal. At the same time, a source light beam is radiated onto the crystal body to obtain a particular wavelength component of the source light beam as a diffracted ray diffracted within the crystal body. Here, the wavelength of the diffracted light is determined by the frequency of the applied acoustic wave, so that the spectrum of the source light beam is obtained by varying the frequency of the acoustic wave and by continuously measuring the intensity of the diffracted ray using a photometer.
The history of the development and advances of the AOTF in recent years is as follows. In 1967, a collinear type was first realized for practical use. Here, the direction in which the acoustic wave travels is the same as the direction in which the light beam travels. However, the most practical and useful AOTFs were not realized until I. C. Chang discovered that TeO.sub.2 is an almost ideal crystal material for manufacturing the AOTF and a non-collinear type AOTF was proposed. In the non-collinear type AOTF, the direction in which the acoustic wave travels intersects with the direction in which the light beam travels. During the past 20 years, hundreds of patents and papers have been disclosed, but almost all of these research and development works are based on early theoretical contributions of I. C. Chang, T. Yano, and A. Watanabe, in which momentum matching and phase matching conditions are commonly accepted.
In the early theoretical research and development, physical models for the AOTF were perfect, but the mathematical analysis always depended on approximation methods. In 1985, Mo Fuqin proposed, for the first time, an accurate mathematical description about the parallel-tangent condition. In 1987, Epikhin gave a general system of equations that represent accurate relationships between acoustic parameters and optical parameters. This is one of the greatest contributions to AOTF design. In 1991, Gass set the acoustic wavevector angle at -80.23.degree. for no particular reason to calculate optimal parameters for the system. In 1992, Ren Quan almost entirely followed the analytic method of Gass's paper and set the acoustic wavevector angle at 105.degree. to calculate a set of parameters for this particular acoustic wavevector angle.
As described above, the AOTF has rapidly spread and progressed in recent years, but a number of problems to be solved remain with prior AOTFs. One of the problems is as follows.
In a prior AOTF, if the intensity of the source light beam is low, or if the intensity of the components of the source light beam in a wavelength region is low, then the accuracy of spectrometry or the accuracy of the finally obtained spectrum becomes low. For example, in the spectral analysis of measuring the absorption spectrum of an object, it is required to accurately measure the intensity of the light in a wavelength region that is absorbed by the object, but the light intensity is often low in the wavelength region due to absorption. Therefore, there has been a problem that the accuracy of the spectral analysis becomes low if an AOTF is used.
In the case where an AOTF is used in a spectroscopic system in which the incident light is weak, the receiving angular aperture becomes an important parameter that determines the accuracy of spectroscopy. As the receiving angular aperture becomes larger, a higher amount of incident light can be collected, so that a higher S/N ratio can be obtained. As proposed by Mo Fuqin, Epikhin, and others, the conservation-of-momentum condition or the parallel-tangent condition has been used in the techniques of determining the position of the crystal body in the AOTF of recent years. In the AOTF designed by using the parallel-tangent condition, an optimal receiving angular aperture can be obtained. The present inventors found through general mathematical analysis that the receiving angular aperture can be set approximately at a maximum value under the conservation-of-momentum condition and the parallel-tangent condition. Further, the present inventors found an optic equivalence incident angle for which the wavelength of the diffracted ordinary ray and the diffracted extraordinary ray becomes identical for an identical acoustic frequency. However, there is still a problem that the receiving angular aperture is not sufficiently large, even if the receiving angular aperture is such an optimal or maximum value.
In a prior AOTF, the source light beam is made perpendicularly incident on the surface of the crystal body (normal incidence). However, there occurs, within the crystal body, a part that is irrelevant of the propagation of the acoustic wave in such an AOTF. This part, called non-diffraction part, becomes greater, as the optic incident angle, which is defined as the angle between the optic axis of the crystal body and the optic axis of the source light beam in the crystal body, is set at a greater value. In fact, when the crystal body is cut in a shape so that the source light beam should be perpendicularly incident on the crystal body surface, the shape has to completely cover the part in which the acoustic wave propagates and also the crystal body surface has to be perpendicular to the optic axis of the source light beam. Therefore, the shape inevitably becomes greater than the shape of the part in which the acoustic wave propagates, so that there occurs within the crystal body a non-diffraction part (idle part) that is irrelevant of the propagation of the acoustic wave.
If the optic incident angle is set at the equivalence incident angle, and the receiving angular aperture is set at a maximum value, then the ratio of the non-diffraction part within the crystal body becomes very large, since the equivalence incident angle is significantly large (for example, about 56.degree. for TeO.sub.2). In this case, if a uniaxial crystal material of the same volume as in the prior AOTF is used, then the propagation part of the acoustic wave becomes small. Therefore, the diffraction length of the acoustic and optic waves becomes short in this case, so that spectral resolution deteriorates. If the diffraction length of the acoustic and optic waves is sufficiently maintained, then the volume of the uniaxial crystal material becomes large, so that the manufacturing cost of the AOTF becomes high.